In recent times, increasing use has been made in glass printing applications of UV hardening organic glass printing inks, which must be subjected to thermal post-treatment after hardening, instead of the previously used ceramic (inorganic) inks. This makes it possible to avoid the use of heavy metals, to gain access to a greater colour range (standard and mixed shades) and to achieve greater brightness and an elevated degree of gloss. Another advantage is the possibility to assess the colour shade directly after UV hardening. In contrast thereto, inorganic inks cannot be assessed until after stoving. They furthermore offer processing advantages, such as a faster heat treatment process (for example approx. 20 min in comparison with 90 min heat treatment) and a reduced heat treatment temperature (for example 160° C. in comparison with 500-700° C.), so reducing processing costs in comparison with processes using inorganic glass printing inks. The organic glass printing inks themselves, however, are more costly than conventional inorganic inks for printing glass.
The milder processing conditions in the printing method with organic glass printing inks ensure that the internal pressure resistance of bottles is not impaired. Organic glass printing inks are, however, not suitable for printing glass which must be post-treated at elevated temperatures, such as moulded glass for automotive applications or glass with high gloss gold print.
With currently available organic printing inks for glass, problems may arise in relation to covering power, mechanical resistance and water resistance. After a glass pretreatment, for example flame treatment, organic printing inks for glass are printed in a two-component printing method, i.e. with a coupling agent to be mixed into the printing ink, and then subjected to thermal post-treatment at temperatures of for example 140-200° C. for a specific period.
It would be desirable to simplify this printing method and so reduce costs. One option for simplification would be to eliminate the requirement to use a two-component printing process, i.e. to use a coupling agent. Another option would be to make thermal post-treatment superfluous.
WO 99/06336 discloses a method for printing glass, in which a glass ink, which comprises a bisphenol A epoxy resin is printed onto a glass substrate and is then hardened by means of radiation, wherein a coupling agent is optionally used.
The inventor found, however, that, without further measures, such a glass printing ink and such a method do not yield either adequate adhesion of the glass ink to the glass substrate or adequate scratch and water resistance. Accordingly, in all the Examples of WO 99/06336, both a coupling agent was used and thermal post-treatment was performed at a temperature of 150° C. to 200° C. U.S. Pat. No. 5,656,336 A discloses a similar process, which also suffers from the same shortcomings.
The present invention is directed to resolving these and other matters.